1. Technical Field
The present invention relates to a technique that reads an output of a light-receiving element such as a photodiode via a reading line.
2. Related Art
In recent years, a display panel in which liquid crystal elements or organic EL elements are arranged in a matrix shape has been widely used for an electronic apparatus such as a cellular phone, a personal digital assistant, or the like. The display panel is used under various environments, for example, under bright conditions where there is sufficient sunlight, that is, daytime, or under dark conditions where external light is insufficient, that is, at night. For this reason, when brightness or image quality of a display element is constant regardless of external light, there is some inconsistency in that, while the display panel can be easily viewed under some conditions, it can hardly be viewed under other conditions. Accordingly, in such a display panel, it is preferable to detect external light and control the brightness or image quality according to the detection result.
In such a control, a light-receiving element such as a photodiode is used to detect external light. In this case, however, if the light-receiving element is provided separately from the display panel, extra space for mounting the light-receiving element is required in the electronic apparatus. Further, an opening portion needs to be provided in the light-receiving element to detect external light.
As a countermeasure against the above-described problems, a technique in which, in the display panel, light-receiving elements and transistors for selecting the light-receiving elements are formed by using the same process as that of transistors for switching pixels in pairs, such that the display panel itself detects external light, has been proposed. Specifically, the pairs of the light-receiving elements and the transistors are arranged at intersections of scanning lines and reading lines respectively. When one scanning line is selected, the transistors in the pairs disposed on the selected scanning line are turned on. Then, the outputs of the light-receiving elements in the corresponding pairs are read via the reading lines.
In this technique, the detection precision of the light amount is degraded due to the noise mixed into the reading line according to the selection of the scanning line. Accordingly, a technique in which the noise is detected and the detected noise is supplied to the reading line after being inverted, such that the noise present on the corresponding reading line is offset, has been proposed (see Japanese Patent Application Publication No. 9-82931).
However, if the transistors or wiring lines are formed in the display panel, a parasitic capacitance appears on various portions, for example, gates of the transistors or the reading lines. For this reason, when an output signal of the light-receiving element is read, the output of the light-receiving element cannot be accurately extracted at the reading line side due to charging and discharging of the gate capacitance or the parasitic capacitance on the reading line.